This invention relates to chart recorders in general and more particularly to an improved chart lock system for an instrument operating under the control of a dedicated microcomputer.
In various types of instruments such as spectrophotometers, a chart is driven in synchronism with a scanning device such as a grating which scans through a range of wavelengths whilst analyzing a sample. Charts used with such chart recorders typically have marked thereon the associated wave number scales or other appropriate calibration. The response of the object under test is plotted as the ordinate value and the change of the scan position plotted on the abscissa. The abscissa markings thus must be synchronized with the drive of the scanning device. This becomes a particular problem where there is a change from the normal scan scaling, i.e., where operation is carried out on an expanded scale for example, or where a time drive is used to scan a particular wave number, for example, over a period of time. After changing modes, to go back to the normal scan, there is a need to accurately reposition the chart to the proper wave number. Without some type of a chart lock, this might require tedious adjustment by the operator and accuracy would then only be determined by his care and ability. The same problem exists when initially setting up the apparatus. Without a chart lock system, the accuracy from the beginning would be determined only by the operator's ability to position the chart.
Previously, instruments of this nature were driven by electromechanical means. The need for a chart lock was recognized and a solution to that problem is disclosed in U.S. Pat. No. 3,396,403 granted to M. A. Ford et al on Aug. 6, 1968. The device disclosed therein uses a mechanical arrangement wherein a spring loaded pin engages an opening in a drive wheel for the chart recorder to insure perfect alignment of a sprocket location, corresponding to a grid line, and the drive. Although this works well in the prior art systems, newer systems utilize microcomputers which have more recently become available. Although the prior art system could be used therein, such use does not fully take advantage of the microcomputer's capabilities.
Thus, it becomes evident that there is a need for a new and improved chart lock system which can be operated in conjunction with an instrument controlled by a dedicated microcomputer which will result in accurate chart positioning and which is easy to operate.